This invention relates to a vertical sync counter for a television receiver and more particularly to a vertical sync counter capable of automatically recognising different TV line standards corresponding to TV systems having different numbers of lines per picture.
Some TV receivers include both a vertical oscillator and a horizontal oscillator. However, for reasons of economy and manufacturing simplicity, some TV receivers have the vertical oscillator replaced with a counter that is driven by the output of the horizontal oscillator. This is possible because of the fixed relationship between the vertical and horizontal frequencies. The number of lines in a TV picture varies throughout the world, but in general there are two main line standards--525 lines or 625 lines. For example, in the United States the N.T.S.C. standard applies wherein each TV picture contains 525 lines. In Europe the P.A.L. and S.E.C.A.M. standards are widely used, each TV picture containing 625 lines. There is a growing commercial need for TV receivers to be capable of recognising and receiving different line standards, particularly with the increased use of video recording and direct satellite broadcasting. Prior art multiple standard TV receivers have used separate decoding circuits for decoding picture information being transmitted at different line standards. However these receivers are costly to manufacture and difficult to adjust and maintain. The problem solved by the present invention is the automatic recognition of different TV line standards.
Accordingly, one object of this invention according to a preferred embodiment thereof is to provide TV circuitry including a vertical sync counter capable of automatically distinguishing between two different TV line standards.
A further object of this invention according to an alternative embodiment is to provide TV circuitry including a vertical sync counter capable of automatically adjusting to any given TV line standard.